Sewage and Its Treatment : experience from setting up sewage treatment plants

Abstract

Growing population has resulted in a steep increase in demand for fresh water coupled with increased contamination from untreated waste water. Along with steps taken to clean our polluted rivers and streams, laws for disposal of waste water are becoming stricter, resulting in an urgent need for setting up facilities for treatment of sewage. There are several treatment options, each with its own set of advantages and disadvantages. Drawing from our experience in setting up and running sewage treatment plants across various locations involving multiple technologies, this paper discusses the major technologies for sewage treatment. 

Key Words

Activated Sludge

Aerobic and Anaerobic

BOD (Biochemical Oxygen Demand) / COD (Chemical Oxygen Demand)

Decomposition

Dosing

Pathogens

Primary, Secondary & Tertiary Treatment

Sewage

Suspended Solids

Water Pollution

Introduction

Sewage, also known as domestic or municipal waste-water, comprises of grey-water (from sinks, tubs, showers, dishwashers, and clothes washers), black-water (the water used to flush toilets, combined with the human waste that it flushes away) along with soaps and detergents and substances that get disposed off in the sewage system, like toilet paper. Sanitary sewers serving industrial areas also carry industrial waste water. Depending on the design of the sewer system, sewage may include run-off water, increasing its volume.

Sewage is characterized by its volume or rate of flow, its physical condition, and its chemical, toxic & bacteriological content. Sewage is mainly bio-degradable and most of it is broken down in the environment. However, the process is slow, and un-treated sewage may contaminate the environment and cause diseases. Sewage may also contain chemical and pharmaceutical substances. Untreated sewage is suspected to be one of the causes of increase in antibiotic resistance in many germs (“superbugs”).

In urban areas as per CPHEEO standards, a single person typically generates around 80 to 120 liters of sewage a day, with a dry sludge component of 50 to 60 g. With increasing population and increasing urbanization, sewage disposal is fast becoming a major sanitary problem. In India, cities, and towns, generate 38,255 kl of waste water daily of which only ~30% (about 11,788 kl) is treated and the rest is left untreated. Apart from having limited sewage treatment facilities, many of treatment facilities that do exist are not functioning properly.

Issues with Untreated Sewage

While untreated sewage contains over 99.5% water, its other contents, namely nutrients (nitrogen and phosphorus), solids (organic and inorganic matter), pathogens (bacteria, viruses and protozoa), helminths (intestinal worms and worm-like parasites), oils and greases, heavy metals (including mercury, cadmium, lead, chromium and copper) and toxic chemicals (including PCBs (polychlorinated biphenyls), PAHs (polycyclic aromatic hydrocarbons), dioxins, furans, pesticides, phenols and chlorinated organics used in bathroom cleaners etc. are responsible for contaminating the environment and spread of disease.

Human exposure to the organisms in sewage-contaminated water takes place usually orally through the mixing of untreated sewage with fresh water, or by skin exposure to contaminated water while swimming, bathing etc. Oral exposure causes gastrointestinal disorders (diseases such as diarrhea, e-coli infections, hepatitis-A, giardiasis, amoebic dysentery, cholera etc.), while skin problems could occur from dermal exposure.

Also, discharges of untreated sewage lead to depletion of oxygen in water bodies which result in the death of aquatic organisms including fish.

The Need to Treat Sewage

The safe treatment of sewage is crucial to the health of any community and maintain the life of aquatic organisms. With increasing population, the improper disposal of domestic sewage and human excreta has posed serious problems for health, environment, and bio-degradation. Untreated sewage often contaminates water bodies (lakes, rivers, and the sea). Sewage contamination increases the concentration of nitrates, phosphates, and organic matter (found in human waste), which serves as a food for algae and bacteria. This causes these organisms to overpopulate to the point where they use up most of the dissolved oxygen that is naturally found in water, making it difficult for the natural fauna of the aquatic environment to live. These bacteria can also lead to spread of water-borne diseases in humans and animals that come in contact with the water. It has been estimated that almost 70% of India’s fresh water sources have been contaminated by sewage, leading to calls for urgent action (e.g. the Ganga Action Plan for Ganga-Yamuna rivers which are among the most polluted in the world).

Environmental Risk of Using Untreated Sewage & Industrial Waste Water

Irrigation with sewage or sewage mixed with industrial effluents results in a saving of 25 to 50% of N and P fertilizer and leads to 15-27 % higher crop productivity. Thus, a significant portion of sewage bypasses any treatment and is used for irrigation or, worse, joins water bodies.

However, there are a number of limitations with regard to waste water treatment and reuse in agriculture, such as the production of waste water when the crops do not require irrigation water, the location of the plants compared to the land requiring irrigation, the match between the waste water fertilizer content and the crop requirements, the risk of over-application, vigorous incidence of weeds and insect pests due to, in general, low uses of pesticides in agroforestry systems and early dropping and softening of fruits, etc. Intensive land application leads to accumulation of salts and heavy metals in the soil, odour problems, salt and colour leaching affecting groundwater and downstream water quality, etc.

This leads to a decline in the land productivity along with a build-up of toxic pollutants, which encourages the overgrowth of weeds, algae, and cyanobacteria and causes groundwater and downstream water quality to deteriorate.(Table – 2).

Benefits of Treating Waste Water

Effective sewage management is essential for nutrient recycling and for maintaining ecosystem integrity. Effective sewage treatment results in separation of sludge and treated water which results in:

1.    Improving the environment through proper drainage and disposal of wastewater

2.    Preventing floods through removal of rainwater

3.    Preserving receiving water quality.

The sludge is a natural organic fertilizer and can be used to restore soil fertility. Treatment and recirculation of treated water from sewage is needed to tackle the growing crisis of a lack of adequate fresh water (Fig – 3). 

Water Guidelines – Legal Requirements

CPCB published discharge limitations for various pollutants including BOD, COD, TSS etc. A snapshot of the General Discharge Limits provided in the CPCB website is provided in Table –1.

In recent times, CPCB, MoEF and permitting authorities are requiring more strict discharge limits.

Many industrial facilities are now required to meet zero-liquid discharge. This means no wastewater discharge is permitted and the industries have to recycle and reuse the treated wastewater. Notification issued by MoEF (Ministry of Environment & Forests) requires that municipal discharges must meet BOD/COD/TSS/TN/TP limit of 10/50/10/10/2 mg/L, respectively, and an effluent coliform limit of 230 MPN/100mL. This implies new technologies need to be installed to achieve new standards.

This paper discusses biological treatment technologies for STPs to achieve effluent BOD/TSS of 30/30 mg/L - which meets the limits prescribed under General Standards by CPCB as shown in the table above. Additional technologies may need to be installed to meet the new limits and to re-use water to meet zero discharge requirements. 

Treatment of Sewage – main technologies*

The most basic treatment of sewage is by collection in a soak-pit and allowing water to soak into the soil. This is usually adopted for independent toilets. As polluting of ground water is a possibility, this treatment is inadequate and currently unacceptable. 

Treatment of sewage is based on a method provided by nature, i.e. by using microbial action. When a steady consistent supply of air is pumped into a tank containing sewage which has been screened to remove all floating debris and non-soluble contents, microbes which are present in it get activated. These microbes are present in the sludge which makes up a substantial part of sewage, and they consume the pollutants in the sewage while the air supply brings them to life and keeps them alive and multiplying. Most aerobic processes for treatment of sewage are variations of the above, “Activated Sludge Process (ASP)”. A sewage treatment plant (STP) based on this aerobic process will consist of the following major stages of treatment:

?      Primary Treatment (Screening): In this stage, raw sewage is screened to remove floating debris/ insoluble impurities such as plastic bags, leaves, twigs, paper etc.

?      Secondary Treatment (Decomposition): In this stage, oxygen (air) is mixed into the sewage to activate the microbes which decompose organics in the sewage and cause them to settle as insoluble sludge (biomass). Water and sludge are separated, the sludge is removed and dried for disposal and the water free from sludge is sent to a clarified water tank.

?      Tertiary Treatment (Treatment of Clarified Water): Clarified water is filtered through a pressure sand filter and an activated sand filter to remove any remaining suspended impurities and a substantial portion of the BOD & COD present in it. Finally, it is disinfected to kill all the bacteria present in it by either chlorination or ozonization or with ultraviolet light. This tertiary treated water can be used to flush toilets, wash roads and yards, and for gardening.

Smaller STPs use variations of the more modern membrane bio-reactor systems (MBR). MBR is a very compact waste treatment system that combines biological decomposition with membrane separation of the sludge (biomass). The membrane compacts & concentrates the sludge, making for a far more compact design than the ASP system described above: it combines the secondary and tertiary treatment into one single step. Further, it produces far less sludge. It is also not so sensitive to input load fluctuations, unlike the ASP system.

The simplest treatment of sewage is the duckweed pond. In this extremely eco-friendly system, sewage is allowed into a constructed water body where certain kinds of aquatic plants are planted which absorb atmospheric oxygen and let this out through their roots thereby providing the oxygen for feeding the microbes which clean up the sewage. This treated water, however, can only be used for gardening, due to the low pathogen removal of this system. Its major drawbacks are that it requires lots of land and can breed mosquitoes; the major advantage is that it requires no electricity to operate if the flow of raw and treated sewage is by gravity.

There are many other specialized technologies, including process variations of the above, which are not yet very popular.

Our Experience in setting up/upgrading/running STPs

RUDIMENTARY TREATMENT METHODS

Constructed Duckweed Pond System 

We constructed a Duckweed Pond System to tackle the problem of untreated sewage from settlements in a semi-rural setup.

The shallow pond, with a depth of ~1.5 m has a retention period of between 7 & 14 days. BOD and SS removal of up to 30 mg/l is achievable in such systems. The duckweed pond has a high mineral and nutrient removal rate due to the uptake of duckweeds.

The principal advantages are a very low cost of construction and low sensitivity to varying flow rates and other seasonal fluctuations. Duckweed is a very hardy plant with simultaneous and significant nutrient removal and yields vegetative matter with a high protein content (40%) which is a good animal feed.

Fish farming in the pond is possible but has not been encouraged for other reasons. Low pathogen removal due to reduced light penetration and high land requirement are the principal disadvantages why the system is only suitable for rural and semi-urban settlements with easy land availability.

We chose to make a duckweed pond to treat raw sewage as the location is outside our lease area and hence the construction of anything with significant time, capital or maintenance requirements was not possible. The overflow of the duckweed pond is further treated in an Effluent Treatment Plant.

Soak Pits

For unconnected toilets in areas with low population, an older technology of soak pits is popular. Initially, individual septic tanks with soak pits were thought to be the right choice. The principal advantage of such a system is ease of construction (especially for widespread colonies with low population). The principal disadvantages are the low rate of pathogen removal, the risk of polluting ground water and inefficient soaking due to unfavourable soil conditions, i.e. impervious rock layer, leading to the overflowing of sewage. The soak pits also need to be cleaned (de-sludged) at regular intervals. Nowadays, soak pits are not allowed due to the risk of polluting ground water, and over time most sewage lines have been/are being connected to the Sewage Treatment Plant.

TECHNOLOGY FOR SMALL STAND ALONE STPs

Packaged STPs

Small-sized Sewage Treatment Plants made of a combination of FRP & Steel are popular options for stand-alone STPs. Such STPs are usually in the form of a compact FRP tank which can be used in a decentralized manner for aerobic treatment of sewage water and is ideal for residential and commercial complexes, hospitals etc.

The STPs incorporate membrane technologies with low operation and maintenance costs and reportedly give a high (90%) reduction in BOD consistently. However, the sludge needs to be handled periodically along with replacement of the (costly) membrane. The initial capital cost is also quite high.

This technology is most suitable where connection to the sewage system is difficult or not possible, and for wastes (like hospital waste) where there is an increased risk of contamination. We had evaluated a project for setting up a packaged STP for a hospital; however, the project is yet to fructify.

STP TECHNOLOGIES ADOPTED FOR MID-SIZED STPs

STPs Based on Membrane Bio-Reactor(MBR) Technology 

The figure above shows various mid sized (~100 kld) Membrane Bioreactor Based STPs constructed at different locations.

Membrane Based Bio-Reactor Systems (MBR Technology)

In this process, sewage decomposition takes place in bio-reactors, i.e. MBR chambers. The bio-reactor comprises of a tank fitted with an aeration grid. The bacterial activity needs dissolved oxygen to synthesize the organic matter. This is supplied by passing air in the form of small bubbles from the bottom of the tank, which increases the contact period and also facilitates mixing, thereby increasing the efficiency of oxygen absorption.

The bacterial population grows on specially designed carrier media (MBR media) which form an integral part of the reactor system. The media are made of small polypropylene elements with a very large surface area for the bacterial population to grow.

The technology allows for rapid treatment and is suitable for small to mid-sized plants with small variations in input sewage load. Maintenance of the membranes and the bacterial growth are very important and thus these plants need to be run with skilled/trained manpower.   

Modular STPs based on MBR technology

TECHNOLOGY ADOPTED FOR MEDIUM TO LARGE SIZE STPs WITH SMALL FOOTPRINT

The Submerged Aerobic Fixed Film Reactor Process (SAFF)

The Submerged Aerobic Fixed Film Reactor Process (SAFF) has a small footprint and effectively treats dilute domestic waste water. The low and stabilized sludge produced eliminates the need for sludge digestion.

The treated water can be used for agriculture and gardening or used to recharge ground water aquifers, while the dry sludge is a good fertilizer in the rural community surrounding our colony.

However, the disadvantage is the need for primary settling to avoid clogging (the process is very sensitive to the quantity of sludge, it can only accept relatively diluted sewage). Another disadvantage could be the relatively costly, proprietary Aerobic Fixed Film which needs proper maintenance and thus skilled manpower. 

This technology was chosen for the said location, as it is suited for congested, sensitive locations: there is no smell and the technology is compact and has lower capital cost than competing technologies.

TECHNOLOGY ADOPTED FOR LARGE SCALE SEWAGE TREATMENT

The Sequencing Batch Reactor Technology

The Sequencing Batch Reactor is a variation of the Activated Sludge process which is a proven and tested sewage treatment process all over the world for the last seven or eight decades. The process requires an uninterrupted power supply for constant aeration and sludge recirculation.

Reactor sludge levels also need to be carefully monitored and excess sludge needs to be withdrawn from the system periodically. Hence, due to its high level of automation, the capital costs of such plants are on the higher side and skilled manpower is required for operation.

However, the process is highly efficient and can remove >90% of the pathogens (bacteria, viruses, faecal coliforms etc.) and BOD. This activated sludge process is operated in batches through auto-control, with aeration and settling in one tank leading to a lower plant foot print. Suspended solid removal is also high.

This process is ideal for larger sewage treatment plants in congested and sensitive locations as there is virtually no smell. This sewage treatment plant was chosen for a colony of 2000 people as the plant needed to be located inside the colony. 

OTHER TECHNOLOGIES

    Trickling Filter

One of the oldest methods of treating sewage, this technology is over 100 years old. Being rugged and simple, the process requires very little monitoring. However, the limitations of the technology, i.e. the need for consistent effluent/sewage input quality, low rate of pathogen removal (from 20% up to 70%), and the risks of blockage of port and bio-filter due to excess biomass growth and the risk of odour and filter fly, rule out the adoption of this process in today’s environment. If adopted at all, it is usually adopted nowadays only for pre-treatment and is followed by Activated Sludge Treatment using any of the technologies described earlier. High Rate Trickling Filters are sometimes adopted when the treated water can be prevented from contaminating drinking water sources and used internally for gardening etc. The sludge needs to be separately treated in a digester and used as manure.

Up-flow Anaerobic Sludge Blanket (UASB)

The Up-flow Anaerobic Sludge Blanket process (USAB) uses anaerobic reactions (in contrast to most of the processes described earlier, which depend on aerobic reactions) to decompose the sludge. UASB reactors are typically suited to dilute waste water streams (3% TSS with particle size >0.75 mm).

Biogas with a high concentration of methane is produced as a by-product, and this may be captured and used as an energy source, whilst forming a blanket of granular sludge which is suspended in the tank. Waste water flows upwards through the blanket and is processed (degraded) by the anaerobic microorganisms. The upward flow combined with the settling action of gravity suspends the blanket with the aid of flocculants.

Since the process handles diluted sewage, sludge handling is minimized. Similarly, power supply interruptions have minimal effect on plant performance and the process can absorb hydraulic and organic shock loading.

However, the USAB process cannot meet the desired effluent discharge standards (as the output effluent is anoxic and invariably exerts a substantial and instantaneous oxygen demand on receiving inland water bodies or when used for irrigation) unless proper post- treatment is adopted (usually through an Activated Sludge based treatment process). Hence this process is not used today except for pre-treatment.

Phyto Remediation and related technologies

Phyto remediation is considered to be a possible method for the removal of pollutants present in waste water and is recognized as a better green remediation technology than many other technologies.

Plants like water hyacinth which grow very rapidly in water bodies are extremely efficient in absorbing nutrients. The quest of such plants for nutrient absorption provides a way for Phyto-remediation of waste water along with the combination of herbicidal control, integrated biological control and watershed management controlling nutrient supply to control plant growth.

Moreover, as a part of solving wastewater treatment problems in urban or industrial areas using this plant, a large number of useful by-products can be developed like animal and fish feed, power plant energy (briquette), ethanol, biogas, composting and fibre-board making.

However, large scale adoption of this relatively cheap and efficient technology is hindered by the risk of the uncontrolled growth of water hyacinth choking up all nearby water bodies.

Soil Bio Technology developed at IIT Bombay is another recent technology used to treat sewage. The VEC patented CAMUS-SBT system produces results of >95% COD reduction using a soil like media in the presence of terrestrial ecology with plants as bioindicators.

Electro-Flocculation

Electro-Coagulation is a system in which contaminants are flocculated by an electrical process into a size which can then be filtered, pressed and disposed of. This flocculation process is based on electrolysis, and so called electro flocculation.

When the installation is switched on, air is blown through the water in the reactor vessel. This is to keep the water in motion and to prevent solids sinking to the bottom. Every thirty seconds an extra boost of “cleaning-air” is blown through the water. In a typical system, the reactor contains Iron (Fe+2) and Aluminium (Al+3) electrodes connected to the cathode, although other types of electrodes can be used. The voltage varies between 9 and 11 volts.

The electrodes connected to the anode are slowly converted into Fe203 and Al2(OH)3. The resulting roughly formed flocks provide a place to which any contaminant in the water can attach. The dissolved contamination is connected to the flocks and is transformed into non-dissolvable particles. These particles can easily be separated from the treated water by filtration.

While the electro-flocculation process can rapidly treat large amounts of waste water, the cost of treatment is prohibitive, mainly due to the cost of the consumable electrodes and electricity. Thus there are no large commercial plants using this technology for treating sewage waste anywhere as on date. 

CONCLUSIONS

Increasing population and rapid urbanization are increasing the stress on our scarce water resources. Compounding the problem multi-fold is the fact that most sources of fresh water – rivers, lakes and ground water are rapidly getting polluted by untreated sewage and industrial waste.

Sewage treatment has thus become of paramount importance and laws on waste water management are becoming increasingly stricter. However, sewage treatment is complicated and has capital and operational costs. In addition, while the basic science of treatment in most treatment philosophies is more or less the same, the requirements of space, capital, and recurring costs vary across technologies. Also, sewage characteristics and uniformity of load vary as do the acceptable level of treatment, rendering some technologies more suitable than others.

Proper selection of the sewage treatment technology is thus very important. This article has described most of the popular technologies adopted for sewage treatment and the possible reasons for their selection. 

REFERENCES

1.    ENVIS Centre on Hygiene, Sanitation, Sewage Treatment Systems and Technology

2.    Dr Arunabha Majumder, STP Technologies & Their Cost Effectiveness, All India Institute of Hygiene & Public Health, Govt. of India

3.    R Kaur, S P Wani, A K Singh and K Lal, Waste water production, treatment and use in India, Indian Council of Agricultural Research, New Delhi, India,

4.    Bhardwaj R M, Status of Waste Water Generation and Treatment in India, IWG-Env Joint Work Session on Water Statistics, Vienna, 20-22 June 2005

5.    CPCB, Status of water supply and waste water collection treatment & disposal in class I cities, 1999, Control of Urban Pollution Series: CUPS/44/1999-2000.

6.    CPCB, Parivesh Sewage Pollution – News Letter, 2005a, Central Pollution Control Board,Ministry of Environment and Forests, Govt. of India, Parivesh Bhawan, East Arjun Nagar, Delhi 110 032,

 https://cpcbenvis.nic.in/newsletter/sewagepollution/contentsewagepoll-0205.htm

7.    Sengupta, A K, WHO guidelines for the safe use of waste water, excreta and grey water, National Workshop on Sustainable Sanitation, 19-20 May 2008, New Delhi. https://www.whoindia.org/LinkFiles/Waste Water_Management_WHO

8.    Guidelines for the safe use of waste water, excreta use of waste water, excreta and grey water, www.indiasanitationportal.org.

9.    CPHEEO Manual, 1993, Central Public Health, Environmental & Engineering Organisation

10. Nayyar, S, Sanitation, Health and Hygiene in India, 2014. (www.healthissuesindia.com)

11.  Mitra, Amarnath, Revolutionizing Sanitary Habits of the “Common” Indian: Story of the “Poop Guy”, IOSR Journal of Business and Management (IOSR-JBM) e-ISSN: 2278-487X, p-ISSN: 2319-7668.

12. UNICEF India, Water, Environment and Sanitation, https://www.unicef.org/india

13. Swachh Bharat Abhiyan, 2014, https://swachhbharat.mygov.in

14. Mungray, Arvind Kumar, Phytoremediation, an option for tertiary treatment of sewage.

15. Saber A. El-Shafai, Fatma A El-Gohary, Fayza A.Nasr. , N. .Peter van der Steen, Huub J. Gijzen, Nutrient recovery from domestic waste water using a UASB-duckweed pond system, March 2006, Bioresource Technology 98 798–80.

16. Ranganathan, S S, Waste Water, India Water Portal, www.indiawaterportal.org

17. CPCB, Status of water supply, waste water generation and treatment In class-I cities & class-II towns of India, Control Of Urban Pollution Series, CPCB website

18. Wikipedia Articles on Sewage and various technologies.

19. Information gathered from discussions with various technology providers and EPC contractors etc. and learning from discussion with Prof. H S Shankar of IIT Bombay on SBT.

Acknowledgements

I would like to place on record my gratitude to my colleagues without whose help, the construction of sewage treatment plants described would not have been possible. I would also like to thank my company for help and support during the construction as well as for the kind permission to put my thoughts based on the experience in the form of this paper. I would also like to thank Mr Chandrashekar Shankar for inputs on SBT and thank Mr Indra N Mitra for reviewing this article and providing immensely valuable inputs.

All thoughts and opinions are my own and do not necessarily represent company policy.